Skim coat application

I watch a lot of overhauling and I see each time they are doing body work it looks as thou they skim coat the entire body then block their cars from there. Is this normal or am I not understanding what they do. Also how heavy or thick is the skim coat put on. I ask this because I am just about finished sanding my 66 dart down to the bare metal which means I hope to at least get it into a coat of primer this weekend then start blocking the car during the winter. I didnt find any past body work on the car so its pretty straight but I want it to be nice when Im done.
Thanks Rod